It is extremely important to reduce memory traffic in graphics processing to lower power usage and/or increase performance.
Position-only shading reduces memory traffic. A position-only shading pipeline includes two pipes, namely the cull pipe and the replay pipe. The triangles enter the cull pipe, and shading of the positions is executed on the vertices. Then follows different types of culling tests, e.g., backface (BF) and view frustum (VF) culling, but also zero coverage culling (discarding triangles that do not cover any samples). Finally, the cull pipe records which triangles survive all culling tests, for example in a bit-stream (one bit per triangle). After that, the replay pipe is executed, and here, full vertex shading is executed only for the vertices of the surviving triangles, followed by the rest of the graphics pipe.